West LA Times

Adversity early in life can have permanent health consequences for people — even if their circumstances improve dramatically later on. Scientists use a cumulative adversity index, or CAI, which quantifies measures of hardship including poverty and stress, to understand health and longevity over the course of an individual’s life. This has been helpful in identifying specific measures governments, healthcare providers, and families can take to improve people’s lives.

Wild animals may also experience adversity early in life, but the effect on their survival and longevity is unknown. While a similar tool could help scientists conserve animal populations by identifying the most influential stressors to mitigate, few populations have been studied over a long enough time to get the data needed to develop a CAI for that species.

UCLA biologists are changing that by creating the first cumulative adversity index for yellow-bellied marmots, based on 62 years of continuous data collection at the Rocky Mountain Biological Laboratory in Colorado. This is the second-longest study of individually marked mammals in the world. The new study, published in Ecology Letters, offers detailed steps for scientists with large datasets for other species to create their own CAI.

The index they developed identified some predictable but also surprising stressors with significant effects on marmot survival and longevity. For example, it was no surprise a late start of the growing season reduced survival because marmots must gain weight during the summer for their 7- to 8-month hibernation. But the finding that summer drought had no effect was unexpected. Predation also played a smaller-than-anticipated role. Not surprisingly, a mother’s death played a large role — but it still did even if it occurred after the pup was weaned. That may be because pups live with their mother for a full year after weaning.

To create the index, doctoral student Xochitl Ortiz-Ross selected data for female marmots born after 2001 — when researchers started quantifying physiological stress — that remained in one of the studied colonies until 2019 to guarantee an accurate record of their pedigree, age and lifetime experiences. Males typically disperse while females remain in the area where they are born so biologists can observe females during their lifespan.

This population of marmots spans a 984-foot (300-meter) elevation change that divides them into up-valley and down-valley groups with different environmental and demographic conditions. The scientists trap individuals biweekly from spring through late summer when marmots are active, collecting behavioral, morphological and physiological data.

Ortiz-Ross identified several ecological, demographic and maternal measures of adversity affecting whether a pup survives its first year: late start of season; summer drought; predation pressure; large litters; male-biased litters; late weaning; poor maternal mass; high maternal stress; and maternal loss. She wanted to find out if these factors had any effect on an individual’s lifespan after its first year.

These variables were fed into computer models that quantified standard, mild, moderate and acute adversity levels. All models yielded similar results: moderate and acute cumulative adversity decreased pup survival odds by 30% and 40%, respectively. Pup survival odds were significantly higher up-valley across all models while maternal loss decreased survival odds in all models by up to 64% in moderate adversity scenarios. Poor maternal mass decreased chances of survival by 77% only under moderate adversity while late weaning decreased odds by 33% only under standardized or raw models. Surprisingly, drought increased odds of survival across all but acute adversity scenarios with the greatest effect observed under moderate adversity.

The average adult lifespan was found to be 3.8 years but acute CAIs tripled adverse effects on life expectancy.

“We found that a CAI effectively captures short-term survival risk in yellow-bellied marmots,” Ortiz-Ross said. “Positive effects didn’t cancel out earlier adverse ones suggesting that adversity does accumulate in marmots.”

The results supported using CAI as a tool to evaluate long-term impacts from multiple early-life stressors on yellow-bellied marmot survival.

“What we’re facing in terms of biodiversity management is death by a thousand cuts,” said Daniel Blumstein co-author and professor of ecology and evolutionary biology at UCLA.“We need a way to figure out which stressors have the biggest cumulative effect.”

Conservation plans targeting this marmot population might focus more on reducing maternal mortality rather than reducing predation or countering summer drought effects since these did not turn out as important as expected.